This invention pertains to the field of chain tensioners, particularly but not exclusively in the automotive field.
In a chain drive, the transmission chain, for example, a chain with rollers, inverted teeth, or another type, is wrapped around a driving sprocket and a driven sprocket. Movement of the stretched or tight strand of the chain is generally controlled by a fixed straight tensioner arm or snubber, while the slack portion of the chain utilizes an adjustable tensioning device to prevent excessive slack in the chain and maintain the path of the chain as it passes around the driving sprocket and over the driven sprocket.
The automobile industry has long been concerned with better tensioning devices. A currently preferred type includes a blade tensioner with a shoe made of plastic material connected to one or more flat springs. The blade tensioner operates by allowing the chain to run across the plastic shoe. The spring blades that are inserted within the shoe cause the shoe to deform to a more arcuate shape as the shoe is heated from the contact of the chain being driven across its surface.
U.S. Pat. No. 5,055,088 discloses such a blade tensioner in which two blade springs are mechanically interlocked within a shoe. U.S. Pat. No. 4,921,472 discloses a blade tensioner in which the plastic shoe is made of rigid heat stabilized nylon. U.S. Pat. No. 5,462,493 discloses a blade type tensioner in which two shoes overlap one or more spring blades.
A conventional blade tensioner of the prior art is illustrated in FIGS. 2, 3, and 4. The shoe element 2 of the tensioner 1 is typically made of thermoplastic material and includes an expanded head or knurl 3 at one end and an expanded head or knurl 4 at the other end. The expanded head 3 and the expanded head 4 have blind pockets 3a and 4a. A group of flat springs, for example, three in the tensioner of FIG. 3, and two in the tensioner of FIG. 4, are connected to the shoe and are indicated as reference 5. The flat springs are shaped substantially as elongated rectangles, with a length slightly less than the distance between the ends of the pockets 3a, 4a. Laterally, the flat springs are contained by lateral containment walls 7; 3b and 4b correspond to heads 3 and 4, and on the opposite side by teeth 6. The head 4 has an aperture 4c to receive a pin (not shown) on which the tensioner 1 is mounted and permitted to move in an arcuate manner.
The tensioner is assembled by inserting the group of flat springs 5, which in the absence of stress have a U-shape, within the shoe 2, which in the absence of stress has an extended or planar shape. Therefore, the group of springs 5 is mounted within the ends of the channels 3a and 4a, and the assembly takes on the shape illustrated in FIGS. 2 and 3. When the tensioner is exposed to heating, which occurs, for example, when it is mounted in the engine of an automobile and the engine is being operated, the plastic material of the shoe deforms additionally and the stretched plastic of the shoe takes on a more curved or arcuate shape, depending on the length of the slack side of the chain. Cyclic variations in the transmitted torque, which are characteristic of the engine in use, can cause continuous variations in the radius of the shoe, resulting in sliding of the springs from the ends between the respective pockets.
One example of the application of a tensioner of the prior art with a shoe is illustrated in FIG. 1, in which the tensioner is indicated at 1, the chain at C, and the sprockets as R1 and R2. R2 is the driving sprocket and R1 is the driven sprocket. The upper side of the chain is the tight strand or side and the lower side is the slack side.
In the prior art, the flat springs 5, as shown best in FIG. 4, have their ends aligned and coplanar with the rest of the body of the blade, or curved in conformance to the concavity of the tensioner shoe. This creates the disadvantage that, after a relatively prolonged working session or after a certain number of cycles of contraction and stretching, the end of the spring, or the group of flat springs, may form a groove in the walls of the pocket of the shoe such as 4a, as indicated with the dotted line 4d. This groove 4d forms a trap for the end of the spring, which impedes its sliding along the pocket 4a and in practice blocks and impedes the tensioner's ability to function optimally.
The purpose of the present invention is to eliminate the disadvantage discussed above of blockage of the ends of the springs in the pockets.